This invention relates to the circulation of a heat carrier fluid through an underground deposit of immobile petroleum, wherein the temperature of the deposit is increased so that the petroleum becomes flowable. Circulation patterns of the heat carrier fluid are controlled so that a washing action strips petroleum values from their locked positions. Petroleum values are removed to an above ground location in part by mixing into the circulating carrier fluid, in part by displacement by the carrier fluid, and in part by the buoyancy of the petroleum values as compared to the heat carrier fluid. Flushing means are provided to control build up of sand accumulations in the well bore.
A considerable amount of attention has been directed in recent years to recovery methods for immobile petroleum. There are numerous major deposits of immobile petroleum known to exist in various parts of the world. These deposits are commonly referred to as tar sands, bitumen deposits, heavy oil deposits and the like. Generally, these deposits have common characteristics such as: the petroleum values have a pour point temperature well above the temperature of the deposit itself; petroleum values fill the available void space within the sand and thus reduce effective permeability to a value near zero; the host sand is unconsolidated and thus provides no effective matrix structure to support the overburden weight when the petroleum values are removed; and the like.
Methods have been developed for recovery of petroleum values in situ and above ground. An in situ method is described in copending application Ser. No. 671,259 of the present inventors, and an above ground method is described in U.S. Pat. No. 3,738,929 of Terry et al.
Particular attention is directed in the present invention to an immobile petroleum deposit that involves an unconsolidated sand. Generally, these deposits contain considerable more petroleum values per unit volume than do the conventional petroleum reservoirs that are located in porous host rock. It is not uncommon to find such a deposit wherein the sand has the appearance of being the intrusive material as compared to a conventional petroleum reservoir wherein the petroleum is the intrusive material that invaded the pore space of the host rock.
It is well known in the art that water preferentially wets rock or sand surfaces in comparison to liquid petroleum. It is not uncommon to find an immobile petroleum deposit in an unconsolidated sand wherein upon close inspection an individual sand grain is surrounded by a thin layer of water, which in turn is surrounded by a thicker layer of immobile petroleum. In these circumstances the sand grain serves as a core with an encasement of water which is further encased with a relatively thick rind of petroleum. Typically the rind of petroleum is at a temperature well below its pour point temperature and thus in effect is frozen in place.
Thus it may be seen that the petroleum values may be dislodged in situ by increasing the temperature of the petroleum to the point where it becomes a flowable liquid compared to its normal state of being a solid encasing rind. It is well known in the art that a petroleum substance can be converted readily from a solid material to a liquid material by the addition of modest amounts of heat, and that the liquid material can be made quite flowable by further additions of heat. Thus the fluidity of the petroleum material can be adjusted by the addition or removal of heat.
Upon drilling a well into an underground deposit of immobile petroleum as described above, little if any of the petroleum values may be recovered by flowing into the well bore. It is easy to envision that by adding heat, by whatever means, in the well bore that the petroleum values may be made flowable and therefore by gravity will flow into the well bore. In the case of an unconsolidated sand as described above, the material flowing into the well bore will include the sand grain core, the encasing water, and the mobilized petroleum matter. The sand grains, with a specific gravity of roughly twice that of water or liquid petroleum, will tend to sink to the bottom of the liquid column, and thus will begin the process of filling the well bore with sand particles, unless the well is equipped with a sand exclusion screen.
In a conventional petroleum reservoir located in competent host rock, the temperature of the reservoir is well above the pour point temperature of the petroleum, and the petroleum is thus readily flowable. Generally the pressure of the reservoir is well above the hydrostatic head pressure. Upon drilling a well into such a reservoir and in the absence of a plug (such as a column of drilling mud) the petroleum will readily flow into the well bore, and at times will flow all the way to the surface. Production may be continued as long as there are flowable fluids under the influence of differential pressure. As the differential pressure declines it is quite common to augment the natural reservoir pressure by injecting a fluid such as water to maintain the reservoir drive. In this manner petroleum is driven by displacement from higher pressure locations to the lower pressure area of the well bore. It is important that that the driving fluid water and the driven fluid petroleum move at essentially the same velocity for an effective sweep of the reservoir. Should the velocity of the driving fluid water significantly exceed the velocity of the driven fluid petroleum due to the difference of the mobility ratios of the two fluids, it is just a question of time until the driving fluid water breaks through to the production well. Upon water break through, the production of petroleum diminishes dramatically and the production of water greatly increases, and in the conventional sense the well is rapidly approaching economic depletion.
In the case of the immobile petroleum deposit, injection of a driving fluid has little effect on the mobility of the petroleum. In the prior art numerous schemes of adding heat to the driving fluid have resulted in failure when attempts were made to drive the petroleum mobilized by the heat to a nearby production well. In other cases the pressure of the hot driving fluid has been increased to a value sufficiently high to fracture the deposit and thus establish communication between the injection well and the production well. As long as this pressure is maintained, the communication passage remains open with large volumes of the driving fluid passing from the injection well, through the fracture, and to the production well. In this arrangement the driving fluid in effect becomes a circulating fluid that bypasses most of the petroleum adjacent to the established underground channel. Upon stopping injection with the attendant decrease in pressure, the unconsolidated nature of the deposit results in slumping into and closing of the communication passage, and termination of production. In some cases production attempts become unsuccessful because of excessive sand accumulations in the well bore of the production well.
It is an object of the present invention to disclose methods that permit continued injection of the heat carrier fluid into the deposit of immobile petroleum for the purposes of adding heat, mobilizing the petroleum, and capturing the petroleum at the surface of the earth. It is a further object of the present invention to disclose methods of controlling sand accumulations in the well bore. Other objects, capabilities and advantages of the present invention will become apparent as the description proceeds and with reference to the accompanying drawings.